1. Field of the Invention
The present invention is directed to a power supply, and more particularly to an inverter AC power supply which includes an output transformer to provide a high frequency resonance voltage for driving one or more loads, such as gas-discharge lamps.
2. Description of the Prior Art
An inverter AC power supply including an output transformer has been known in the art as disclosed in U.S. Pat. Nos. 4,719,556 and 5,130,610. The prior art power supply comprises an inverter providing a high frequency AC voltage to a primary winding of the output transformer and an L-C resonant circuit for providing a resonant current to a load which is coupled across a secondary winding of the output transformer. This prior art power supply is intended to limit, by the use of the output transformer, voltage appearing at receptacles or connection between an output end of the power supply and a load in order to avoid accidental electrical shocks even if personnel should touch electrode of the receptacle at the time of connecting the load to the power supply. The L-C resonant circuit is formed by a tuning capacitor connected across the primary winding and a tuning inductor connected in series with the primary winding. Since the tuning capacitor and inductor are both connected to the primary winding of the output transformer, there is a problem that, when the load is disconnected from the secondary winding, the L-C resonant circuit is kept active and therefore the circuit responds to act to operate the inverter at a high frequency near a resonant frequency of the L-C circuit in an attempt to provide a high starter voltage even in the absence of the load. This accompanies correspondingly increased resonant voltage and current in the circuit, which necessitates the use of circuit elements of high dielectric strength and capacity that are generally costly and bulky and are therefore not preferred in the design of the power supply. In order to avoid this problem, it is proposed to includes a clamping circuit to prevent undue increase of the resonant voltage in the absence of the load. However, the clamping circuit inherently adds circuit complexity and therefore cost to the power circuit. Another U.S. Pat. No. 4,392,087 discloses an inverter AC power supply for gas-discharge lamp which includes an inverter and a leakage output transformer having a primary winding connected to the output of the inverter and a secondary winding connected across a load or lamp. In this circuit, a tuning capacitor is connected in parallel with the load across the secondary winding and is cooperative with the leakage inductance on the secondary winding of the output transformer to form a resonant circuit on the side of the secondary winding for supplying a resonant current to the lamp. However, since the leakage transformer is inherently of bulky configuration, the power supply is difficult to be fabricated into a compact design. In the meanwhile, there is a potential problem for the inverter AC power supply in that a noise is inherently produced due to high frequency switching of the inverter, and in that a circuit board mounting the circuit component is likely to make a stray capacity between a conductor pattern of the board and a casing of the power supply. When the stray capacity becomes critical, the noise is easy to transmit back to the inverter input. Therefore, when the inverter is powered by a commercial AC line or power mains through a suitable AC-to-DC converter as is usual with the inverter AC power supply, the inverter may bring about a considerable noise problem on the power mains. In view of the above, the leakage inductance in the circuit of U.S. Pat. No. 4,392,087 is not expected to block the feedback noise, since the leakage inductance is located on the secondary winding of the transformer and not on the primary winding in direct electrical connection to the inverter.